The present invention relates to a power-assisted steering system for use in wheeled vehicles and more particularly to a control apparatus for the power-assisted steering system which includes a steering shaft arranged to be rotated by steering effort applied thereto, a source of fluid under pressure in the form of a fluid pump driven by a prime mover of the vehicle, a hydraulic cylinder having a power piston mounted for reciprocating movement in the cylinder and operatively connected to driven members of the steering system, and a servo valve interconnecting the fluid pump and the cylinder and responsive to rotary motion of the steering shaft for selectively directing the flow of fluid under pressure from the pump to opposite fluid chambers of the cylinder to effect reciprocating movement of the power piston.
In such a conventional power-assisted steering system as described above, the pressure of fluid discharged from the pump varies in accordance with resistance acting on the steerable road-engaging wheels of the vehicle. When the steering shaft is rotated to its rotary end to move the power piston to its stroke end, the pump load rapidly increases to increase the fluid pressure up to a relief pressure. Under such a condition, it has been experienced that the prime mover becomes unstable in its operation or suddenly stops due to rapid increase of the pump load.